The present invention relates to devices for positioning sensors, particularly proximity sensors used in turbines to detect thrust bearing wear.
Despite their seeming simplicity of operation, turbines employed for the generation of electric power are complex devices comprising a number of subsystems. To achieve efficient conversion of the energy of an elastic fluid, such as steam, into mechanical rotation, specially machined and balanced turbine rotor buckets are positioned between stationary, fluid directing diaphragms. Because the elastic fluid is typically at greatly elevated temperatures, the design of the turbine system must take into account the thermal expansion that occurs during machine startup. Additionally, means must be provided to control axial movements of the rotor shaft. To ensure that the energy bearing elastic fluid is directed onto the buckets from the diaphragms, specially designed brass labyrinth seals are provided between successive bucket stages. If the axial motion of the rotor shaft exceeds certain limits, then damage to the brass seals will likely result. While the seals are replaceable, it is not only expensive to replace them but also requires shutting down of the power plant for a period of time. To protect against axial motion of the turbine rotor assembly, a thrust bearing is typically provided at a position intermediate the low-pressure and high-pressure stages of a turbine. The rotor assembly is typically provided with a collar rotating with the shaft and used as an axial reference for the shaft position. The axial position of the turbine rotor shaft is an important variable in the operation of the power plant since a variation of only a few hundreds of an inch in a device tens of feet long can result in damage to the labyrinth seals. While it is possible to factor into the design tolerances which account for thermal expansion, certain improper modes of boiler operation can result in water injection into the turbine system, thereby creating large axial forces on the turbine rotor. It is these forces that can produce thrust bearing wear, damage to the seals and, ultimately, plant shutdown.
Presently, thrust bearing wear detectors are employed to sense the position of the rotating collar fixed to the thrust bearing assembly. If the position indicated by the sensor exceeds a fixed limit, a cautionary indication is typically provided to the system operator. If the indicated position exceeds still larger limits, an automatic system trip may occur, in which case the automatic control system typically cycles the plant through a shutdown sequence. This is a precautionary measure, which while protective of the equipment, is costly with respect to lost generation capacity. Accordingly, it is of great value to be able to verify the correct operation of the thrust bearing wear detector system at predetermined or random time intervals. Thus, the proximity sensor which is typically an electronic device, must be treated not only for mechanical positioning, but the electrical circuitry used in connection therewith should also be tested to determine that it too is functioning properly. For it must be remembered that the thrust bearing wear detection subsystem is one that will be operated and relied upon for periods of several years and longer without human intervention.
For adequate tests of the proximity sensor it is therefore necessary to be able to move the sensing device toward and away from the thrust bearing collar a fixed, predetermined and precise distance reliably over a period of years.
One form of positioning device for a turbine rotor position sensor is described in U.S. Pat. No. 3,989,408 issued Nov. 2, 1976 to Karl O. Jaegtnes. There is apparently disclosed therein a piston-within-a-piston arrangement for sensor positioning. However, this arrangement is unnecessarily complicated and provides minimal piston areas against which a motive fluid such as air or oil can act. This reduced area therefore requires higher fluid pressures than are necessary. It further appears to necessitate the use of piston seals, failure of which renders the positioning device inoperative for its intended purposes. Moreover, the use of seals coupled with the relatively low area provided for the motive fluid to work against, increases the probability that one or both of the pistons will stick causing failure of the apparatus to perform its verification function. Additionally, the piston-within-a-piston arrangement of Jaegtnes requires a complex fluid channel configuration requiring the conjunction of an outer passage in the casing with an inner passage through the intermediate piston in order to effect a movement of the innermost piston. This structure is necessitated by this piston-within-a-piston arrangement. In addition to the operational problems that result from a seal failure of the Jaegtnes device, the use of seals potentially limits the life of the device by employing a component that is susceptible to wear and to a relatively early failure.